Rice, as a grain, presents some very difficult problems, unlike most other grains, as the rice is processed from the time it is in the field and reaching maturity, to the time it is ready for wholesale or retail distribution and consumption.
A principal reason for processing problems, unique to rice, is the particular nature of the rice kernels and particularly the starchy endosperm of the kernel. The endosperm has a flinty nature which is another way of saying that the kernel may be considered to be brittle and rather easily fractured. The rice kernels are quite hygroscopic so as to readily absorb moisture, and the rice kernels also give up their moisture relatively easily. The rice kernels have a propensity to shatter or fissure quite easily, and when this occurs, the individual kernels break down and result in the formation of a powder or flour. In any event, fissured or shattered kernels have a very low value in the marketplace as does the resulting powder or flour, as compared to the whole kernels.
This fissuring or shattering of the rice kernels occurs principally during rapid changes in moisture content of such rice kernels. The rapid absorbing of moisture and the rapid drying of the kernels will both tend to cause fissuring. When mature rice stands in the field just prior to harvesting, if climatic conditions induce rapid drying, fissuring may very likely occur to a large percentage of the kernels; and in a somewhat similar way, if such standing rice is relatively dry and is suddenly subjected to high moisture conditions, a significant proportion of the kernels of rice may fissure due to the rapid absorption of moisture. Fissuring is also likely to occur during artificial drying after harvest.
An excellent discussion of drying rice may be found in "Rice Chemistry and Technology", edited by D. F. Houston, published by American Association of Cereal Chemists, Inc., St. Paul, Minn., Copyright 1972, at pp. 140-165.
As a practical matter, substantially all drying of rice that is done in the United States is done by commercial dryers in fixed installations located adjacent substantial storage capacity provided by grain elevators. Such drying is in continuous flow heated air dryers by a multi-pass method which essentially means that rice is run through a dryer wherein the rice kernels pass downwardly through columns and simultaneously hot air is blown transversely through the columns of moving rice. After the rice has passed through the dryer, the rice is returned to storage bins for tempering, and after a tempering of four to twenty-four hours, the rice is cycled through the dryer again. Each time the rice is passed through the dryer, the drying process may take fifteen to thirty minutes and may remove two to three percentage points of moisture. It can readily be understood that this method of drying rice is rather cumbersome and requires a substantial amount of handling of the rice between the dryer and the storage facility.
Some commercial drying of rice is in a column type dryer, and other rice dryers require the falling rice to pass downwardly over a series of baffles to simultaneously mix the moving rice as it is dried.
In any event, this multi-pass method of drying rice is substantially more cumbersome than the methods used in drying other grain. It has been experienced that because of the extreme abrasive nature of rice as it is handled in a multi-pass drying operation, the drying equipment is extremely subject to wear and the major parts of the machinery which are in constant engagement with the continually moving rice must be frequently replaced or rebuilt.
Because of the peculiar nature of rice and the propensity of it to shatter or fissure, there has been practically no drying of rice accomplished by farmers in the United States on their own farms. Of course, in other countries, individual farmers may dry their rice by laying the rice out on concrete slabs in the sun, and to some extent, rice may be dried in shallow beds with air being forced upwardly through it. Of course, such techniques do not accommodate large quantities of rice.
The fissuring problem of rice has been and continues to be a major obstacle to efficient and thorough drying of rice for maximum market value.
Techniques and apparatus which have been successfully used in drying other grains such as corn, oats, wheat, etc., has not been successful in drying rice. None of these other grains is subject to fissuring and loss of market value as is rice. The typical technique in drying corn, which is not successful in drying rice, is to simply drive as much hot air at as high a temperature as can be attained in order to dry the corn as rapidly as possible. If this technique used for corn were to be tried with rice, the whole batch of rice would be completely fissured and almost valueless.
One other grain, soybeans, is known to present problems in drying which are of the same general nature as presented by rice, especially when the soybeans are being processed for use as seed.